Send document: new option "Save mail in dossier"

:construction: 
The send document functionality should be extended with a new option "save copy in the dossier":
![image](https://cloud.githubusercontent.com/assets/485755/3223342/9bf010a8-f021-11e3-894c-099bd0e044e4.png)

Todos:
 - New checkbox (not activated by default)
 - Mail saving
 - Translations
 - Tests